Merck Manual

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Brain Aneurysms

(Cerebral Aneurysms)


Ji Y. Chong

, MD, Weill Cornell Medical College

Reviewed/Revised Jul 2020 | Modified Sep 2022

An aneurysm is a bulge (dilation) in the wall of an artery. Aneurysms that occur in the arteries of the brain (cerebral arteries) are called cerebral aneurysms.


Many cerebral aneurysms result from a weakness in the artery wall that is present at birth (congenital). Others are caused by atherosclerosis Atherosclerosis Atherosclerosis is a condition in which patchy deposits of fatty material (atheromas or atherosclerotic plaques) develop in the walls of medium-sized and large arteries, leading to reduced or... read more Atherosclerosis (buildup of plaque or fatty material in the wall of blood vessels). People may have only one cerebral aneurysm or several. Others result from bacterial or fungal infections in the wall of the artery that develop after recreational use of intravenous drugs such as heroin. Such infections usually start elsewhere in the body, typically in a heart valve Infective Endocarditis Infective endocarditis is an infection of the lining of the heart (endocardium) and usually also of the heart valves. Infective endocarditis occurs when bacteria enter the bloodstream and travel... read more Infective Endocarditis , before spreading to the wall of the artery.


Most cerebral aneurysms do not cause symptoms unless they are large or rupture.

Larger unruptured cerebral aneurysms can push on brain tissue and nerves and cause headache, which may feel pounding in time with the pulse (pulsatile). Less often, people may have dilated pupils and/or symptoms of a stroke, such as weakness or paralysis on one side of the body.

Rupture of a cerebral aneurysm causes immediate, severe headache that is sometimes described as coming on like a thunderclap. If the ruptured aneurysm bleeds into the brain tissue (intracerebral hemorrhage Intracerebral Hemorrhage An intracerebral hemorrhage is bleeding within the brain. Intracerebral hemorrhage usually results from chronic high blood pressure. The first symptom is often a severe headache. Diagnosis is... read more ), people often develop a symptoms of a stroke Symptoms A stroke occurs when an artery to the brain becomes blocked or ruptures, resulting in death of an area of brain tissue due to loss of its blood supply (cerebral infarction) and symptoms that... read more (which is typically caused by blood clots rather than bleeding). The bleeding can increase the pressure in the brain, which can lead to coma and sometimes death.


  • CT or magnetic resonance angiography

Because cerebral aneurysms are near the brain and are usually small, their diagnosis and treatment differ from those of other aneurysms.

Infected aneurysms of the cerebral arteries are particularly dangerous, making early treatment important. Diagnosis of an infected aneurysm is made by CT angiography, blood tests that indicate increased levels of inflammation (such as an elevated C-reactive protein level or an elevated erythrocyte sedimentation rate), and blood cultures (samples of blood that are grown in a laboratory) that show the growth of microorganisms (such as bacteria or fungi).


  • Surgical or catheter-based repair

Treatment of cerebral aneurysms often involves surgical repair of the aneurysm. Surgical repair involves placing a clip to close off the aneurysm. The clip is placed where the aneurysm pouches out from the main artery.

Sometimes a less invasive treatment (endovascular coiling) can be done. Endovascular coiling involves inserting a small but long flexible plastic tube (catheter) into an artery in the thigh. The catheter is then pushed through the arteries of the body to the aneurysm in the brain artery. Small metal coils are injected through the catheter into the opening of the aneurysm to block blood flow into it, thereby allowing the aneurysm to shrink.

When the aneurysm is infected, antibiotics or antifungal drugs are given.

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